This invention relates to fingerprint recognition systems and particularly fingerprint sensing devices for use in such systems.
Fingerprint recognition has been suggested for many applications, ranging from high security uses such as access control for buildings, computers or the like to low security uses such as replacements for conventional locks and keys. The main advantages of such systems are that they are easy and convenient to use, avoid the need for keys, personal identification numbers and the like, and should be less susceptible to fraudulent use. The sensing device is an important part of the system and the quality of the representation of the fingerprint that the device picks up will affect recognition capability and the amount of preprocessing required for verification.
Conventional forms of fingerprint sensing devices rely on optical detection methods. A simple optical sensing method can be open to fraudulent use by presenting a photographic image of a fingerprint. A more secure and common approach is to use a glass prism with frustrated total internal reflection. Light is directed through one face, reflected at a second face and passes out at the third face. A finger is placed on the second face and at the points where the finger is in contact with the glass, i.e. at the ridges of the fingerprint, reflection no longer takes place. Light continues to be reflected at regions where troughs of the fingerprint pattern are present. Light output from the third face is picked up by an image sensor. A binary image, in which those parts of the fingertip in contact with the glass show as black and the rest as white, can therefore be obtained. Such an optical sensing device has disadvantages however. For example, the device is comparatively bulky. Also it is essential that the contact surface be maintained clean and free of dirt or grease. Moreover, problems can occur because some people have drier fingers than others and because most people's fingers become drier in cold weather. A dry finger actually touches the glass at many fewer points than a moist finger with the result that the lines of the fingerprint appear as rows of small dots so that more image processing becomes necessary.
An alternative kind of fingerprint sensor was proposed in U.S. Pat. No. 4,353,056 which uses a capacitive sensing approach. The described sensor has a two dimensional, row and columm, array of capacitors, each comprising a pair of spaced electrodes, carried in a sensing member and covered by an insulating film. The sensors rely upon deformation to the sensing member caused by a finger being placed thereon so as to vary locally the spacing between capacitor electrodes, according to the ridge/trough pattern of the fingerprint, and hence, the capacitance of the capacitors. In one arrangement, the capacitors of each column are connected in series with the columns of capacitors connected in parallel and a voltage is applied across the columns. In another arrangement, a voltage is applied to each individual capacitor in the array. Sensing in the respective two arrangements is accomplished by detecting the change of voltage distribution in the series connected capacitors or by measuring the voltage values of the individual capacitances resulting from local deformation. To achieve this, an individual connection is required from the detection circuit to each capacitor.
While the described sensor may not suffer form the problems associated with the kind of sensor employing an optical sensing technique, it suffers from its own problems. For example, because the sensor relies in operation on deformation and requires the use of resilient materials, problems with durability and reliability may be experienced. Moreover, the need to provide a respective connection to each individual capacitor in the array means that a very large number of connecting lines is necessary. This creates difficulties, both in the fabrication of the sensing member and its interconnection with the detection circuit. In practice the large number of connections would likely also cause operating difficulties due to stray capacitances.